The invention relates to a method for controlling a radio unit in a radio system comprising at least a first radio unit and a second radio unit, in which method at least one of the radio units communicates on a direct mode channel.
The invention relates to the field of mobile telephone systems. A subscriber in a mobile telephone system, i.e. a subscriber station, such as a radio telephone or other mobile unit, may be connected to a radio network or radio system via system and traffic channels maintained by base stations of the radio network.
In addition to system channels provided by mobile telephone networks, the radio system may employ what are known as direct mode channels. In other words, the communication mode is direct. When operating in direct mode, the subscriber stations do not communicate directly with the radio network or its base stations. Direct mode channels are frequencies at which radio telephones or other mobile stations can communicate independently of the system directly with one another.
Direct mode channels are typically employed in situations where, for example, a number of hand-held mobile telephones communicate with one another at such a distance from a base station that system channels cannot be used.
Another important utility for direct mode channels consists in increasing system capacity when the traffic load on the system increases rapidly in some part, for instance a spotlike part, of the system coverage area.
The term direct or simplex channel or simplex connection is used for a direct mode channel. A direct mode channel is a channel that is typically not available to the system. It is not a frequency pair, but a single channel having the breadth of one of the system channels, e.g. 12.5 kHz or 25 kHz. Of the radio telephones operating on a direct mode channel, the sender has tuned his or her transmitter to a channel and transmits speech or data information. The other radio telephones set in a mode of using direct mode channels have tuned their receivers to the same channel and thus hear the transmission directly.
On a direct mode channel, it is possible to operate by analog modulation or digital modulation. A radio telephone transmitting on the channel may also transmit signalling information, such as information on user access rights and priorities or on a given group communicating on the channel. Encryption may be employed on the direct mode channel or plain speech may be transmitted.
Subscriber stations, i.e. radio units, operating in the direct mode, i.e. on a direct mode channel, communicate with other subscriber stations on a direct mode channel without being in direct communication with base stations of the radio network. However, subscriber stations in the direct mode can communicate with the radio network via repeater stations.
Prior art solutions typically operate in such a manner that the system does not check the access rights of radio units communicating on a direct mode channel, since typically subscriber stations communicating on a direct mode channel do not communicate with the mobile telephone system and its subscriber authentication units. Thus, in the prior art systems, radio units unauthorized to communicate in the mobile telephone system can communicate on direct mode channels alongside the authorized radio units. Such unauthorized radio units may be, for example, stolen mobiles or units prohibited from use for some other reason, for instance on account of the fact that the holder of the radio telephone has not paid his her or its bill to the telephone company.
Thus the problem with the prior art solutions is that subscriber stations, i.e. radio units, not authorized to communicate in the system can nevertheless communicate on a direct mode channel.